This invention relates to signaling apparatus for indicating an open circuit condition of an electric circuit interrupter such as a blown electric fuse. More particularly, this invention relates to signaling apparatus of the aforementioned type which is used on enclosed fusible electrical apparatus and more specifically, on multiphase enclosed fusible electrical apparatus.
It is known in the art to provide a signaling device such as a lamp in series circuit with a semiconductor switch across an electrical source ahead of a fuse and to connect a control circuit for the semiconductor switch across the same source on the load side of the fuse. When the fuse blows, current flow through a control electrode and one switching electrode of the semiconductor switch renders that switch conductive for energizing the lamp to indicate the open condition of the fuse.
It is also known to provide enclosed fusible electrical apparatus with one or more low current neon lamps mounted on or through a wall of the enclosure and to provide each of these lamps with an external, high ohmic resistance for limiting current through the lamp. The lamp and external resistance are connected in parallel with the fuse and a shunting resistor is connected between opposite terminals of fuses in different phases of the protected apparatus. When a fuse blows, a circuit is completed through the shunting resistor and the respective series resistor with the lamp to energize the lamp.
The known circuits described above operate in response to detected current flow. Additionally, none of the known circuits address a potentially hazardous situation created when unprotected conductors of the indicator circuit are connected at the opposite terminals of the fuse, and particularly the conductor connected to the supply side of the fuse. If this conductor should become damaged and come into contact with a conductive portion of the apparatus, or if it is pinched in the enclosure upon closing the doors or cover thereof, the apparatus or the enclosure will be directly shorted to the supply current with no protective means for interrupting that short circuit.